The present invention relates generally to tensioners used with chain drives in automotive timing and power transmission applications. In particular, the present invention is related to a chain tensioner device which tensions two separate strands of chain equally.
Chain tensioning devices, such as hydraulic tensioners and blade-type tensioners, are used as control devices for power transmission chains as the chain travels between a plurality of sprockets. In an automotive application, the tension of the chain can vary greatly due to the wide variation in the temperature and the linear expansion among the various parts of the engine. Moreover, wear to the chain components during prolonged use can produce a decrease in the tension of the chain. As a result, it is important to impart and maintain a certain degree of tension to the chain to prevent noise, slippage, or unmeshing of the chain with the sprocket teeth. It is especially important in the case of a chain-driven camshaft in an internal combustion engine to prevent the chain from slipping because the camshaft timing can be misaligned by several degrees, possibly rendering the engine inoperative or causing damage.
Hydraulic chain tensioners typically have a plunger slidably fitted into a chamber and biased outward by a spring to provide tension to the chain. A lever arm is often used at the end of the plunger to assist in the tensioning of the chain. The hydraulic pressure from an external source, such as an oil pump or the like, flows into the chamber through passages formed in the housing. The plunger is moved outward against the arm by the combined efforts of the hydraulic pressure and the spring force.
When the plunger tends to move in a reverse direction (inward) away from the chain, typically a check valve is provided to restrict the flow of fluid from the chamber. In such a fashion, the tensioner achieves a so-called no-return function, i.e., movements of the plunger are easy in one direction (outward) but difficult in the reverse direction. An example of a check valve is shown in Dusinberre, II et al., U.S. Pat. No. 5,989,139, which is incorporated herein by reference. In addition, rack and ratchet mechanisms, which are well known in the art are employed to provide a mechanical no-return function.
One example of a chain tensioner which uses a hydraulic tensioner and a pivoted lever to tension a chain is described in Sato et al., U.S. Pat. No. 5,318,482. Sato et al. show a conventional hydraulic tensioner with a plunger pressing a pivoted lever against a chain to impart an appropriate tension to the chain. The tensioner and single arm of Sato et al. has certain limitations, however, in the amount of chain slack it can take up during the life of the chain. In addition, the single shoe arm of Sato et al. has limitations in the ability to absorb and damp cyclic vibrations in the chain during operation.
An example of a blade-type tensioner which uses a plastic shoe biased by a blade spring is described in Turner et al., U.S. Pat. No. 3,490,302. Turner et al. show a blade spring mechanically interlocked to a plastic shoe. During operation of the engine, the spring causes the shoe to gradually assume a more arcuate shape which imparts tension to an associated chain or takes up increased slack as the chain wears. Due to the nature of the shoe, however, the tensioner has a slow reaction time in response to changes in chain tension and a limited ability to respond to cyclic vibrations.
Conventional prior art tensioners which tension only one strand of chain, i.e., the length of chain between two sprockets, in an engine timing application have a common weakness. During operation of the engine, wear on the various chain parts causes the chain to lengthen. Taking up the resulting slack on one side of an engine timing system and not the other can cause the timing of the camshaft to change relative to the crankshaft.
Other prior art chain tensioners impart a load to both strands of timing chain by a pair of pistons housed in a common housing located between the chain strands. However, the two pistons in these tensioners act independently and unequally upon the tight and slack strands of chain and fail to address the change of timing which occurs when one piston travels farther and takes up more slack on one side of the timing chain during operation and wear.
To address the above problems the present invention includes a single actuator operating two coupled shoes or wear faces simultaneously. The faces bear against two separate strands of the same chain. This provides potentially double the operating take-up for a given range of tensioner operation as compared to a conventional hydraulic tensioner acting upon a single arm with a pivot. When used to tension separate strands of a single chain, vibrations which occur in one strand of chain tend to be cancelled when the energy of those vibrations are transferred to or combined with those in another strand through the multistrand tensioner. Further, when taking up chain slack in an engine timing application, the present invention helps prevent changes in the timing between the crankshaft and the camshaft during operation and as the chain wears and slackens.